Impact: remove unnecessary code path

Block pc requests always use bio and rq->data is always NULL.  No need
to worry about !rq->bio cases in idefloppy_block_pc_cmd().  Note that
ide-atapi uses ide_pio_bytes() for bio PIO transfer which handle sg
fine.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>